DESCRIPTION

This module generalises the mechanism of the wantarray function, allowing a function to determine in some detail how its return value is going to be immediately used.

Top-level contexts:

The three kinds of top-level context are well known:

VOID

The return value is not being used in any way. It could be an entire statement like foo();, or the last component of a compound statement which is itself in void context, such as $test || foo();n. Be warned that the last statement of a subroutine will be in whatever context the subroutine was called in, because the result is implicitly returned.

Lvalue subroutines:

The introduction of lvalue subroutines in Perl 5.6 has created a new type of contextual information, which is independent of those listed above. When an lvalue subroutine is called, it can either be called in the ordinary way (so that its result is treated as an ordinary value, an rvalue); or else it can be called so that its result is considered updatable, an lvalue.

These rather arcane terms (lvalue and rvalue) are easier to remember if you know why they are so called. If you consider a simple assignment statement left = right, then the left-hand side is an lvalue and the right-hand side is an rvalue.

So (for lvalue subroutines only) there are two new types of context:

RVALUE

The caller is definitely not trying to assign to the result:

foo();
my $x = foo();
...etc...

If the sub is declared without the :lvalue attribute, then it will always be in RVALUE context.

If you need to return values from an lvalue subroutine in RVALUE context, you should use the rreturn function rather than an ordinary return. Otherwise you'll probably get a compile-time error in perl 5.6.1 and later.

LVALUE

Either the caller is directly assigning to the result of the sub call:

foo() = $x;
foo() = (1, 1, 2, 3, 5, 8);

or the caller is making a reference to the result, which might be assigned to later:

my $ref = \(foo()); # Could now have: $$ref = 99;
# Note that this example imposes LIST context on the sub call.
# So we're taking a reference to the first element to be
# returned _in list context_.
# If we want to call the function in scalar context, we can
# do it like this:
my $ref = \(scalar foo());

or else the result of the function call is being used as part of the argument list for another function call:

This prints $x = ; $y = ; $z = 3, which may not be what you were expecting. The reason is that the assignment is in scalar context, so the comma operator is in scalar context too, and discards all values but the last. You can fix it by writing (list) = (1,2,3); instead.

If your lvalue subroutine is used on the left of an assignment statement, it's in ASSIGN context. If ASSIGN is the only argument to want(), then it returns a reference to an array of the value(s) of the right-hand side.

In this case, you should return with the lnoreturn function, rather than an ordinary return.

This makes it very easy to write lvalue subroutines which do clever things:

You can check this using conditionals like if (want('CODE')). There is also a function wantref() which returns one of the strings "CODE", "HASH", "ARRAY", "GLOB", "SCALAR" or "OBJECT"; or the empty string if a reference is not expected.

Because want('SCALAR') is already used to select ordinary scalar context, you have to use want('REFSCALAR') to find out if a SCALAR reference is expected. Or you could use want('REF') eq 'SCALAR' of course.

Be warned that want('ARRAY') is a very different thing from wantarray().

Item count

Sometimes in list context the caller is expecting a particular number of items to be returned:

my ($x, $y) = foo(); # foo is expected to return two items

If you pass a number to the want function, then it will return true or false according to whether at least that many items are wanted. So if we are in the definition of a sub which is being called as above, then:

want(1) returns true
want(2) returns true
want(3) returns false

Sometimes there is no limit to the number of items that might be used:

my @x = foo();
do_something_with( foo() );

In this case, want(2), want(100), want(1E9) and so on will all return true; and so will want('Infinity').

The howmany function can be used to find out how many items are wanted. If the context is scalar, then want(1) returns true and howmany() returns 1. If you want to check whether your result is being assigned to a singleton list, you can say if (want('LIST', 1)) { ... }.

Boolean context

Sometimes the caller is only interested in the truth or falsity of a function's return value:

If the REF keyword is the only parameter passed, then the type of reference will be returned. This is just a synonym for the wantref function: it's included because you might find it useful if you don't want to pollute your namespace by importing several functions, and to conform to Damian Conway's suggestion in RFC 21.

Finally, the keyword COUNT can be used, provided that it's the only keyword you pass. Mixing COUNT with other keywords is an error. This is a synonym for the howmany function.

A full list of the permitted keyword is in the ARGUMENTS section below.

rreturn

Use this function instead of return from inside an lvalue subroutine when you know that you're in RVALUE context. If you try to use a normal return, you'll get a compile-time error in Perl 5.6.1 and above unless you return an lvalue. (Note: this is no longer true in Perl 5.16, where an ordinary return will once again work.)

lnoreturn

Use this function instead of return from inside an lvalue subroutine when you're in ASSIGN context and you've used want('ASSIGN') to carry out the appropriate action.

If you use rreturn or lnoreturn, then you have to put a bare return; at the very end of your lvalue subroutine, in order to stop the Perl compiler from complaining. Think of it as akin to the 1; that you have to put at the end of a module. (Note: this is no longer true in Perl 5.16.)

howmany()

Returns the expectation count, i.e. the number of items expected. If the expectation count is undefined, that indicates that an unlimited number of items might be used (e.g. the return value is being assigned to an array). In void context the expectation count is zero, and in scalar context it is one.

The same as want('COUNT').

wantref()

Returns the type of reference which the caller is expecting, or the empty string if the caller isn't expecting a reference immediately.

ARGUMENTS

The permitted arguments to the want function are listed below. The list is structured so that sub-contexts appear below the context that they are part of.

VOID

SCALAR

REF

REFSCALAR

CODE

HASH

ARRAY

GLOB

OBJECT

BOOL

LIST

COUNT

<number>

Infinity

LVALUE

ASSIGN

RVALUE

EXPORT

The want and rreturn functions are exported by default. The wantref and/or howmany functions can also be imported:

use Want qw'want howmany';

If you don't import these functions, you must qualify their names as (e.g.) Want::wantref.

INTERFACE

This module is still under development, and the public interface may change in future versions. The want function can now be regarded as stable.

I'd be interested to know how you're using this module.

SUBTLETIES

There are two different levels of BOOL context. Pure boolean context occurs in conditional expressions, and the operands of the xor and !/not operators. Pure boolean context also propagates down through the && and || operators.

However, consider an expression like my $x = foo() && "yes". The subroutine is called in pseudo-boolean context - its return value isn't entirely ignored, because the undefined value, the empty string and the integer 0 are all false.

At the moment want('BOOL') is true in either pure or pseudo boolean context. Let me know if this is a problem.

BUGS

* Doesn't work from inside a tie-handler.

AUTHOR

Robin Houston, <robin@cpan.org>

Thanks to Damian Conway for encouragement and good suggestions, and Father Chrysostomos for a patch.